When forming materials each having a different refractive index, it is inevitable that there is a change of reflectivity within a certain wavelength range (FIG. 1). The structure formed in such a manner is referred to as a distributed Bragg's reflector (DBR).
In particular, the DBR technique can make the reflectivity to be 100% or 0% in a desired wavelength according to the refractive index difference and the thickness of each layer. Thus, the technique has been introduced to mainly fabricate filters and reflectors used in current optical devices such as camera lenses. A device, which is manufactured by applying the DBR technique to fabricate a semiconductor laser resonator, is referred to as a vertical resonator surface-emitting device (see H. Li, K. Iga, “Vertical Cavity Surface-Emitting Laser Devices,” Springer, Berlin, 2002). The device emits a light in a direction perpendicular to a plane forming the semiconductor active layer. Such device has been often utilized as a light source in connection with small optical communication and optical recording apparatuses due to its low production costs, as well as its low optical spreading and optical coupling.
The key components of the vertical resonator device are DBRs, which are formed relative to the semiconductor active layer. DBRs are generally fabricated by laminating dielectric materials each having a different refractive index or by laminating compound semiconductors, which are lattice-matched to a substrate and have differing compositions.
For example, in case of a commercially available vertical resonator surface-emitting device having a mean wavelength of 0.98 μm, DBR is prepared by laminating a pair of AlGaAs/GaAs. Also, DBR can be prepared by laminating a pair of InGaAlAs/InGaAlAs in case of 1.3 to 1.55 μm broadband vertical resonator surface-emitting device, wherein the former InGaAlAs and the latter InGaAlAs have differing compositions. However, the mean reflection wavelength of DBR changes too easily, even when there is a slight change in the composition and thickness.
In a large area semiconductor, the composition of materials is subjected to DBR changes due to an unequal heating of the substrate, thereby decreasing uniformity. Furthermore, since the composition and thickness must be controlled each time DBR is fabricated, there is needed a substantial effort to maintain the conditions of apparatus for fabricating the same, which obviously decreases productivity.